CRITICAL LIMITS OF SOIL PENETRATION RESISTANCE IN A ...

Moacir Tuzzin de Moraes et al.

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288

CRITICAL LIMITS OF SOIL PENETRATION RESISTANCE IN A

RHODIC EUTRUDOX(1)

Moacir Tuzzin de Moraes(2), Henrique Debiasi(3), Reimar Carlesso(4), Julio Cezar

Franchini(3) & Vanderlei Rodrigues da Silva(5)

SUMMARY

Soil penetration resistance is an important indicator of soil physical quality

and the critical limit of 2 MPa has been widely used to characterize the soil physical

quality, in both no-tillage and conventional systems. The aim of this study was to

quantify the influence of different tillage and cropping systems on the soil

penetration resistance in a Rhodic Eutrudox. The experiment was carried out in a

5 × 2 factorial, completely randomized block design (tillage systems vs cropping

systems), with four replications. The tillage systems consisted of: conventional

tillage disk harrow; minimum tillage with annual chiseling; minimum tillage with

chiseling every three years; no-tillage for 11 consecutive years; and no-tillage for

24 consecutive years. The factor cropping systems was represented by: crop

rotation and crop succession. The soil penetration resistance (SPR) was determined

in 20 soil samples per treatment and layer (0.0-0.10; 0.10-0.20 and 0.20-0.30 m) for

each soil matric potential: -6, -10, -33, -100, -500 kPa. The SPR was determined at a

volumetric soil water content equivalent to the fraction of plant-available water of

0.7. There were no differences of soil penetration resistance between the two

cropping systems. Differences in soil penetration resistance among tillage systems

were related to the matric potential at which the samples were equilibrated. The

critical SPR limit of 2 MPa normally used for conventional tillage should be

maintained. However, this value of 2 MPa is inappropriate for the physical quality

characterization of Rhodic Eutrudox under no-tillage and/or minimum tillage with

chiseling. Regardless of the cropping systems, the critical SPR limit should be

raised to 3 MPa for minimum tillage with chiseling and to 3.5 MPa for no-tillage.

Index terms: no-tillage, soil chiseling, level compaction.

(1) Part of the Master's dissertation of the first author submitted at Federal University of Santa Maria (UFSM) and Embrapa Soja.Received for publication February 26, 2013 and approved September 19, 2013.

(2) Doctoral student, Post-Graduation course in Soil Science, Federal University of Rio Grande do Sul - UFRGS. Av. BentoGonçalves, 7712, Prédio 41506. CEP 91540-000 Porto Alegre (RS), Brazil. E-mail: [email protected]

(3) Researcher, Embrapa Soja. Rod. Carlos João Strass, Distrito de Warta. Caixa Postal 231. CEP 86001-970 Londrina (PR), Brazil.E-mail: [email protected], [email protected]

(4) Professor, Department of Rural Engineering at the UFSM, Rural Sciences Center - CCR, Campus UFSM. Rua Q, 68, Camobi.CEP 97105-900 Santa Maria (RS), Brazil. Email: [email protected]

(5) Professor, Department of Environment and Agronomy Science, UFSM. Campus de Frederico Westphalen. Linha Sete deSetembro, s/n. Rod. BR 386, km 40. CEP 98400-000 Frederico Westphalen (RS), Brazil. E-mail: [email protected]

CRITICAL LIMITS OF SOIL PENETRATION RESISTANCE IN A RHODIC EUTRUDOX 289

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RESUMO: LIMITES CRÍTICOS DE RESISTÊNCIA À PENETRAÇÃO EM UMLATOSSOLO VERMELHO DISTROFÉRRICO

A resistência do solo à penetração é um importante indicador da qualidade física do solo,e o limite crítico de 2 MPa vem sendo utilizado para caracterizar a qualidade física do solo,tanto em sistema plantio direto como em cultivos convencionais. Objetivou-se com este trabalhoverificar a influência de diferentes sistemas de manejo do solo e modelos de produção sobre aSPR em um Latossolo Vermelho distroférrico. O experimento foi conduzido em um delineamentode blocos ao acaso, em esquema fatorial 5 × 2 (manejos do solo × modelos de produção), comquatro repetições. Os manejos do solo foram: sistema preparo convencional; sistema preparomínimo escarificado a cada ano; sistema preparo mínimo escarificado a cada três anos;sistema plantio direto contínuo por 11 anos; e sistema plantio direto contínuo por 24 anos. Ofator modelo de produção foi composto por: rotação e sucessão de culturas. A resistência do soloà penetração foi determinada em 20 amostras indeformadas de solo por tratamento e porcamada (0,0-0,10; 0,10-0,20 e 0,20-0,30 m), as quais foram equilibradas nos potenciaismatriciais de -6, -10, -33, -100 e -500 kPa. Foi determinada a resistência do solo à penetraçãono conteúdo de água volumétrico equivalente à fração de água disponível às plantas de 0,7.Não houve diferenças de resistência do solo à penetração entre os modelos de produção. Adetecção de diferenças de resistência do solo à penetração entre os sistemas de manejo do solofoi dependente do potencial matricial de água no solo, em que as amostras foram equilibradas.O limite crítico de resistência do solo à penetração usualmente utilizado de 2 MPa deve sermantido para o sistema preparo convencional. Todavia, esse valor foi inadequado para acaracterização da qualidade física do Latossolo Vermelho distroférrico, sob sistema plantiodireto e, ou, no sistema de preparo mínimo. Independentemente do modelo de produção, oslimites crítico de resistência do solo à penetração devem ser ampliados para 3 MPa, no sistemapreparo mínimo com escarificação, e para 3,5 MPa, no sistema plantio direto.

Termos de indexação: sistema plantio direto, escarificação do solo, nível de compactação.

INTRODUCTION

No-tillage (NT) has been increasingly used due tothe numerous economic and agronomic advantagessuch as soil and water conservation and improved cropyield (Silva et al., 2012). However, some reports inliterature have shown that NT has led to the formationof a layer characterized by a high compaction level,generally located between 0.10 and 0.20 m deep inthe soil (Franchini et al., 2009). Apart from increasingsoil resistance to root penetration (Moraes et al., 2012,2013) and limiting the depth and volume of soilexplored by plant roots for water and nutrients(Bergamin et al., 2010), soil compaction reduces totalporosity, macroporosity, aeration, infiltration capacity(Dias Junior & Pierce, 1996), and saturated hydraulicconductivity (Silva et al., 2009).

Soil penetration resistance (SPR) has been usedby several researchers to quantify the soil quality andto identify the layers with increased degree ofcompaction (Franchini et al., 2011; Moraes et al.,2013). In making management decisions to solve soilcompaction problems under NT, fixed SPR valuesconsidered limiting have been proposed and used(Reichert et al., 2007; Betioli Júnior et al., 2012),regardless of the soil type or tillage system. Althoughthe most commonly used value is 2 MPa (Tormena etal., 1998; Silva et al., 2008; Lima et al., 2012), recentresearch results have shown the possibility ofincreasing the limiting value of SPR to 3.5 MPa under

consolidated NT conditions, due to the presence ofcontinuous and biological pores, which favor rootgrowth even in areas with low SPR (Tormena et al.,2007; Betioli Júnior et al., 2012). However, there arestill doubts about which SPR level ought to be usedas critical or limiting under long-term NT, becausein areas where the measured SPR reflects a high levelof soil compaction, the grain yield is not affected. Thisshows that these SPR thresholds may be inadequate.Therefore, it is possible that the limiting values ofSPR to root growth vary according to the tillage system.

Soil management practices to control soilcompaction with periodic plowing and chiseling havebeen tested by several authors (Tavares Filho et al.,2006; Silva et al., 2012). However, the residual effectof these interventions on soil physical propertiesusually disappeared after a few crop cycles (Drescheret al., 2011), sometimes in less than six months (Silvaet al., 2012) or after one year (Tavares Filho et al.,2006). Another measure that has been recommendedto improve the physical quality of compacted soilsinvolves the adoption of cropping systems that includeplants with a high biomass production potential andcharacterized by an abundant, deep and aggressiveroot system (Franchini et al., 2011).

Although the effect of crop rotation on soil physicalquality in NT has been the subject of several studies(Genro Junior et al., 2009; Lanzanova et al., 2010;Debiasi et al., 2010; Costa et al., 2011), there are stilldoubts about the efficiency of this practice in


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mitigating soil compaction. This is because thebenefits of crop rotation on soil physical quality arenot always detectable, as most of these studies wereconducted on a short- or medium-term basis,regardless of the time adoption of the NT.

Therefore, the objectives of this study were toevaluate the effects of different tillage and croppingsystems on SPR in a Rhodic Eutrudox with veryclayey texture and identify the need to alter the criticallimit of SPR of 2 MPa for the evaluation of soil physicalquality as a function of soil management.

MATERIAL AND METHODS

Study site

This long-term experiment was established in1988 at the Experimental Station of EmbrapaSoybean, in Londrina, State of Paraná, SouthernBrazil (Lat. 23o 11’ S; Long. 51° 11’ W; 620 m asl).According to the Köppen classification, the regionalclimate is Subtropical Humid, Mesothermal (Cfa),with a mean annual temperature of 20 oC and1,622 mm rainfall. The soil of the study area isbasaltic and was classified as Latossolo Vermelhodistroférrico (Brazilian soil classification) (Santos etal., 2013), or Rhodic Eutrudox (American soilclassification) (Soil Survey Staff, 2010) with very clayeytexture.

Experimental design and treatments

The experiment had a 5 × 2 factorial design (tillagesystems x cropping systems), distributed in arandomized block design with four replications. Thetreatments consisted of the following tillage systems:conventional tillage with heavy plowing to a depth of0.15 m, then light harrowing before each winter andsummer growing season (CT); minimum tillage withannual chiseling (MTC1); minimum tillage withchiseling every three years (MTC3); continuous NTfor 11 years, established in 2001 (NT11); andcontinuous NT for 24 years, established in 1988(NT24). Between 1988 and 2001, the soil in NT11 wastilled with a moldboard plow (average working depthof 0.32 m), followed by harrowing before planting thesummer crop, and heavy harrowing (average workingdepth of 0.15 m) followed by light disking beforeplanting the winter crop. The MTC1 and MTC3 plotswere chiseled before planting the winter crops, usinga mounted chisel plow with rollers and four shanksspaced 0.40 m apart, working at an average depth of0.30 m and an angle of 45º. The two cropping systemswere: wheat succession (Triticum aestivum L.) inwinter and soybean (Glycine max (L.) Merr.) insummer and, four-year crop rotation system, with thefollowing species: white lupine (Lupinus albus L.) orradish (Raphanus sativus L.)/maize (Zea mays L.);white oat (Avena strigosa Schreb.)/soybean; wheat/

soybean; and wheat/soybean in winter/summerrotation, respectively. The 30 × 10 m plots were spaced7 m from each other to ensure tractor turning duringoperations. The average dry biomass production of thespecies in succession and crop rotation systems wasapproximately 5.3 and 7 Mg ha-1 yr-1, respectively.The average initial soil organic carbon content inthe 0.0-0.10 m layer was 18.9, 19.9, 19.8, 20.6 and21.9 g kg-1 in CT, MTC1, MTC3, NT11 and NT24,respectively. Details of the soil physical and chemicalcharacteristics of the site before the establishment ofthe experiment were reported by Piccinin (2005).

Soil sampling

The soil was sampled 10 and 22 months after thelast chiseling in MTC1 and MTC3 plots, respectively.Soil profiles were opened in each treatment betweencrop rows during summer (soybean) and a total of600 undisturbed samples were collected from the layers0.0-0.10, 0.10-0.20 and 0.20-0.30 m with core samplers(internal diameter 5.0 cm, height 5.0 cm). Thesamples were collected at soil moisture content nearfield capacity, with a soil sampler device coupled to atractor, to ensure that the core samplers werevertically inserted and the samples sequentiallycollected from the center of each soil layer.

Determination of soil physical, mechanicaland hydraulic properties

The 600 soil samples were divided into five groupsof 120 soil samples, totalizing eight undisturbedsamples per tillage systems and layers, regardless ofthe cropping systems. Samples were saturated andsubjected to the following matric potential of water insoil (Ψ): -3 and -6 kPa on a tension table (Embrapa,1997) and -10, -33, -100, and -500 kPa, using Richards’pressure plate apparatus. After reaching equilibriumat each water tension, the soil samples were weighedand with the exception of Ψ of -3 kPa, the sampleswere subjected soil penetration tests to determine theSPR, using a static penetrometer (Marconi, model MA933) consisting of a metallic rod (diameter 4 mm,base area 0.1256 cm2, cone half angle of 30o andpenetration rate of 20 mm min-1). The soil sampleswere oven-dried at 105 oC for 24 h to quantify thesoil bulk density (BD) and volumetric soil watercontent (θ) (m3 m-3).

The gravimetric soil water content at Ψ of -1500 kPawas determined in disturbed soil samples passedthrough a 2-mm sieve with a dew-point psychrometer(model WP4-C) (Klein et al., 2006). Thus, θ at this Ψ(-1500 kPa) was determined from the gravimetric soilwater content and BD.

Using the SPR curve of each tillage systems, asdescribed by Moraes (2013), the SPR was determinedat θ equivalent to the fraction of plant-available water(PAW) of 0.7. The PAW is the ratio of the actual andthe potential water storage capacity of the soil (Santos& Carlesso, 1998). For the potential water storage


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capacity of the soil, the values of è between fieldcapacity (FC), at a Ψ of -10 kPa, and permanentwilting point (PWP), at a Ψ of -1500 kPa, wereconsidered. The θ values for Ψ between FC and PWPwere estimated using the water retention curve foreach tillage system, as described by Moraes (2013).

The results of SPR and θ were subjected to analysisof variance (p<0.05), separately for each layer(0.0-0.10, 0.10-0.20 and 0.20-0.30 m) and Ψ (-6, -10, -33, -100, -500 kPa). When the treatment effects weresignificant, means were compared by Tukey’s test at5 % probability. Data analyses were performed usingthe computer program Statistical Analysis System(SAS, 2002).

RESULTS AND DISCUSSION

The interaction between the soil managementsystems and cropping systems was not significant forany of the variables in the three layers of the soilprofile. Thus, the simple effect of each of the factors(tillage and cropping systems) was compared in thelayers 0.0-0.10, 0.10-0.20 and 0.20-0.30 m.

Clearly, the volumetric soil water content (θ)decreased with the increase in matric potential (Ψ)(Figure 1-I). There was no significant influence ofcropping systems on water retention in all layersevaluated, however the increasing trend of SPR as afunction of Ψ can be attributed to the reduction in θ,which agrees with the exponential increase in SPRdue to the reduction of the gravimetric soil watercontent (Moraes et al., 2012, 2013).

The effects of the cropping systems on SPR weresmall, regardless of Ψ (Figure 1-II). The SPR wassignificantly higher in the crop rotation thansuccession system in the 0.0-0.10 m layer at Ψ of-6 kPa (Figure 1a-II), and in the 0.20-0.30 m layer atΨ of -33 kPa (Figure 1c-II). At the other Ψ values,SPR was not affected by cropping systems. This resultwas consistent with the absence of significant effectsof cropping systems on the other soil physicalproperties evaluated (Moraes, 2013), showing thatSPR, a property considered highly sensitive to soilstructural alterations induced by differentmanagement practices (Abreu et al., 2004), must notbe used to differentiate between crop rotation andsuccession. Abreu et al. (2004) evaluated the breakingof compacted layers by chiseling or plants with anaggressive root system and concluded that theresulting improvements in soil physical conditionsdepend on the physical property used. Based on soilhydraulic conductivity, these authors observed thatbiological chiseling of the soil was more effective thanmechanical chiseling, with the opposite result whenusing SPR as indicator.

In the layers below 0.10 m, it was observed that atΨ of -10 kPa (field capacity), both the crop rotation

and succession systems reached the critical SPR levelof 2 MPa (Figure 1a-II). This shows that at soil watercontents below field capacity, based on the criterionof SPR of 2 MPa, the physical conditions would berestrictive to crop growth and development.

Except at Ψ of -10 kPa, θ was influenced by themanagement systems (Figure 2-I). In general, themanagement systems with greater tillage intensity(CT and MTC1) resulted in lower values of θ than theother treatments, which was most evident in the0.0-0.10 m layer. These differences in the θ valuesmay have resulted in variations in SPR between thesoil management systems. Furthermore, irrespectiveof the management system and Ψ, there was anincrease in θ with soil depth, suggesting the existenceof alterations in the pore size distribution within thesoil profile.

The SPR values were influenced by the variationof θ at each Ψ value and by the soil managementsystems (Figure 2-II). The reduction in SPR due tothe increase in θ is reflected in the Ψ values. Thisreduction in SPR at high water contents is possiblythe result of the presence of water which facilitatespenetration of the rod by its lubricating action betweenthe particles (Assis et al., 2009). Furthermore, anincrease in θ causes reduction of friction as well ascohesion forces between soil particles and aggregates,resulting in a decrease in SPR (Ros et al., 2011).

At Ψ, -6 and -10 kPa (Figure 2a, b-II), the averageSPR values of the tillage systems were close to 2 MPa.However, reduction of Ψ below -33 kPa (Figure 2c-II)resulted in SPR above 2 MPa, with the exception ofthe 0.0-0.10 m layer in the CT treatment. Even whendetermined at high θ (field capacity), the SPR valueswere close to or greater than 2 MPa in all tillagesystems under NT, including in the recently tilled(MTC1 and MTC3), demonstrating clearly that 2 MPais inappropriate as critical threshold for this RhodicEutrudox.

In the 0.0-0.10 m layer, the SPR values were lowerin treatment CT than in the other tillage systems(Figure 2-II) at virtually all Ψ values, demonstratingthe effect of disruption of the soil structure by the useof a disk harrow. In this layer, on the other hand,there were no differences between the times of adoptionof NT (NT11 and NT24) compared with MTC1 andMTC3. This indicates that the effects of soil chiselingpersisted less than 10 months.

In the 0.10-0.20 m layer, SPR was in most caseshigher in CT than MTC1, clearly demonstrating theeffect of the formation of a “tillage pan” below a depthof 0.10 m in CT. In the same layer, no significantdifferences in SPR were observed between treatmentsMTC1 and MTC3, in all situations evaluated (Figure2c,e-II), however, it is noteworthy that, in bothsituations, the θ of MTC1 was lower than MTC3(Figure 2c,e-I), which can contribute to markeddifferences in SPR between the treatments (Moraeset al., 2012). Similarly, there were no differences in


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SPR values between the no-tillage periods, showingno increase in the compaction degree over time (Figure2-II). This indicates that the soil physical quality ofNT can be maintained over time without requiringmechanical soil tillage.

In the 0.10-0.20 m layer, the differences betweenthe SPR values in treatment MTC1 and the NTsystem (NT11 and NT24) were altered as a function ofΨ (Figure 2-II). At Ψ of -500 kPa (Figure 2e-II), SPRwas significantly lower in MTC1 than NT11. The MTC1resulted in lower SPR values in relation to NT24 at Ψof -6, -10 and -100 kPa (Figure 2a,b,d-II). This showsthat in this layer, the reduction in SPR by soilchiseling was detectable up to 10 months afterapplication. On the contrary, there were no significantdifferences between MTC3 and continuous no-tillage(NT11 and NT24), except at Ψ of -100 kPa, where SPRwas significantly higher in NT24 (Figure 2d-II). Thus,22 months after soil chiseling, it was impossible todetect residual effects of this practice on SPR.

In the 0.20-0.30 m layer, significant differenceswere also observed for SPR results among themanagement systems as a function of Ψ (Figure 2-II). In all cases, there were no differences in SPRbetween treatments with periodic soil chiseling (MTC1and MTC3). Similar results were obtained betweenthe period of NT installation (NT11 and NT24).Comparing periodic soil chiseling (MTC1 and MTC3)with no-tillage systems (NT11 and NT24), significantdifferences were observed only when SPR wasdetermined at Ψ of -100 kPa (Figure 2d-II). In thiscase, the SPR values were higher in NT24 than in

MTC1 and MTC3. The absence of differences in SPRbetween periodic soil chiseling (MTC1 and MTC3) andcontinuous no-tillage (NT11 and NT24) demonstratesthe uselessness of periodic soil chiseling to reduce thecompaction degree of this soil, since the effect persistsfor less than 10 months and soybeans and wheat yieldsare not increased compared to NT (NT11 and NT24),as reported by Moraes (2013) in a study conducted atthe same location.

The existence of a soil layer with a greater degreeof compaction, a “tillage pan” in CT was identifieddue to the sharp increase in SPR in the 0.20-0.30 mlayer in relation to the surface layer (Figure 2-II).However, in this 0.20-0.30 m layer, the variations inSPR in CT plots in relation to other tillage systemsare dependent on the varied values of θ at different Ψ.The SPR values in CT, evaluated at Ψ equal to -6 kPa,were higher than in all soil management. However,at other Ψ values, no significant differences wereobserved in the 0.20-0.30 m layer between treatmentsCT and NT (NT11 and NT24), indicating that thereduction of θ may have altered the sensitivity of SPRto detect any increase in compaction level in treatmentCT in relation to NT11 and NT24. Probably the absenceof differences in the SPR values in NT24 comparedwith CT at Ψ greater than -6 kPa may be related tosoil structure formation in NT24, in which theaggregate bonding strength was higher, as well ashaving BD lower than CT (Moraes, 2013).

Concerning the possibility of raising the criticallimits of SPR for this very clayey Oxisol, it wasobserved that the values of θ at which SPR reaches 2

Cropping system

Figure 1. Volumetric soil water content (I) and soil penetration resistance (II) as function of cropping

systems, determined at matrix potential of -6 kPa (a), -10 kPa (b), -33 kPa (c), -100 kPa (d) and -500 kPa

(e). ns Not significant by the F-test (p<0.05). Means followed by the same letter in the same layer do not

differ by the Tukey test (p<0.05). Vertical bars indicate standard deviation from the mean.


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and 3.5 MPa were altered depending on the tillagesystems (Figure 3-I) and independent of the croppingsystems (Figure 3-II). In the 0.0-0.10 m layer, the èat SPR of 2 MPa was equal to θ at Ψ of -10 kPa andhigher than that at Ψ of -33 kPa in treatment NT11.There were no significant differences between theθ values at Ψ of -33 kPa and the θ values at SPR of2 MPa in MTC3 and NT24 (Figure 3a-I). The θ valueat SPR of 3.5 MPa was less than θ values at Ψ of-33 kPa in all tillage and cropping systems (Figure3a-I,a-II). Also in this layer, it was observed that θvalues at SPR of 2 MPa was equivalent to θ valuesat Ψ of -33 kPa in both crop succession and rotationtreatments (Figure 3a-II). The θ values at SPR of3.5 MPa in CT did not differ from the θ values at Ψ of-1500 kPa, indicating that for this soil structuralcondition, an increase in the critical SPR value from2 to 3.5 MPa cannot be recommended. However, inthe 0.0-0.10 m layer, the θ values at SPR of 3.5 MPawere higher than at Ψ of -1500 kPa in all tillagesystems (Figure 3a-I).

In the 0.10-0.20 m layer, θ at SPR of 2 MPa washigher than the water content at Ψ of -10 kPa intreatments MTC3 and NT11. However, θ at Ψ of -10 kPawas identical with q at SPR of 2 MPa in the 0.10-0.20 mlayer of treatments CT, MTC1 and NT24 (Figure3b-I). This indicates that the critical threshold of 2MPa SPR cannot be considered an appropriate valuefor any of the tillage systems, since this valuesuggests, regardless of the tillage system, that θmust be greater than or equal to field capacity (Ψ of-10 kPa) to prevent root growth restrictions. In thesame layer, the θ at SPR of 2 MPa was greater thanθ at Ψ of -33 kPa in all tillage and cropping systems,and in some cases lower (CT, MTC1, NT24 and undercrop succession and rotation) or higher (MTC3 andNT11) than the amount obtained at Ψ of -10 kPa(Figure 3b-I,b-II). Again, this indicates that in alltillage and cropping systems, the use of a criticalthreshold of 2 MPa SPR would already result in limitedplant growth and development, however, thislimitation was not reflected in the soybean and wheat

Tillage system

Figure 2. Volumetric soil water content (I) and soil penetration resistance (II) as a function of tillage systems,

based on the matrix potential of -6 kPa (a), -10 kPa (b), -33 kPa (c) -100 kPa (d) and -500 kPa (e). ns Not

significant by the F-test (p<0.05). Means followed by the same letter in the same layer do not differ by

Tukey’s test (p<0.05). CT: conventional tillage system; MTC1: minimum tillage, annual chiseling; MTC3:

minimum tillage, chiseled every three years; NT11: no-tillage for 11 years; NT24: no-tillage for 24 years.

Vertical bars indicate the standard deviation from the mean value.


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yields of this cropping season (Moraes, 2013) nor inthe grain yield recorded over two decades in thisexperiment (Franchini et al., 2012). The at SPR of3.5 MPa, regardless of tillage and cropping systems,was lower than the water content at Ψ of -10 kPa,indicating that this threshold ensures better soilphysical conditions for plant growth and development.For the same Oxisol, Betioli Júnior et al. (2012)reported that a SPR value of 2 MPa overestimatedthe limitations for root growth and development in acontinuous NT system, while a critical SPR greaterthan 2 MPa resulted in a non-limiting water rangeconsistent with the soil physical quality under NT.

In the 0.20-0.30 m layer, the θ at Ψ of -10 kPa waslower than the value obtained at SPR of 2 MPa intreatments MTC3 and NT11, but equal to the valueobtained in the treatments CT, MTC1 and NT24 atthe same SPR (Figure 3c-I). The θ at Ψ of -10 kPasuction was lower than the value obtained at SPR of2 MPa under crop rotation, however, it was equal tothe value obtained at SPR of 2 MPa under cropsuccession (Figure 3c-II). These results indicate thatthe use of a critical SPR value of 2 MPa for all tillage

and cropping systems in this layer is related to strongphysical restrictions to root penetration, which couldreduce the extraction of water and nutrients by plants.Therefore, by increasing the critical limit of SPR from2 to 3.5 MPa, we observed that the soil water contentin all tillage and cropping systems was lower thanthat obtained at Ψ of -10 kPa (Figure 3c-I,c-II). Thus,using a critical SPR limit of 3.5 MPa in the 0.20-0.30 mlayer indicates adequate soil physical conditions toextract water and nutrients and no physical barriersto root penetration, thus corroborating the results ofsoybean and wheat grain yield (Moraes, 2013) andalso yield stability in a long-term NT system studiedby Franchini et al. (2012).

Analyzing the Ψ when SPR value reached 2 and3.5 MPa in the three layers evaluated, a significantinfluence of the tillage systems (Figure 4-I) but notthe cropping systems was observed (Figure 4-II). Inthe 0.0-0.10 m layer, the Ψ values in the MTC3, NT11and NT24 managements were close to -33 kPa at SPRof 2 MPa. In these tillage systems, extending thecritical limit of SPR to 3.5 MPa resulted in Ψ near-150 kPa (Figure 4a-I). The change of SPR from 2 to

Tillage system Cropping system

Figure 3. Volumetric water content at field capacity (matric potential of -10 and -33 kPa), permanent wilting

point (matric potential of -1500 kPa) limits and soil penetration resistance at 2 and 3.5 MPa, in the 0.0 -

0.10 m (a) 0.10-0.20 m (b) and 0.20-0.30 m (c) layers under different tillage (I) and cropping systems (II)

in a Rhodic Eutrudox. Means followed by the same letter, uppercase between tillage systems, and

lowercase in the same tillage or cropping systems, do not differ by the Tukey test (p<0.05). CT:

conventional tillage system; MTC1: minimum tillage, annual chiseling; MTC3: minimum tillage, chiseled

every three years; NT11: no-tillage for 11 years; NT24: no-tillage for 24 years. The vertical bars indicate

the standard deviation from mean value due to tillage and/or cropping systems.


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3.5 MPa in the 0.0-0.10 m layer in treatments CTand MTC1 was inadequate because the Ψ increasedfrom -260 to -920 kPa and -108 to -460 kPa,respectively, indicating that, in soils degraded by theannual intensive tillage with disk harrow (CT), thecritical limit of SPR should be maintained at 2 MPa,while in MTC1, the limiting value of SPR should beless than 3.5 MPa.

In the 0.10-0.20 m layer and considering a SPRvalue of 2 MPa, the Ψ values in all tillage systemswere higher (less negative) than -18 kPa, as observedin MTC1 (Figure 4b-I). The determination of SPRat 3.5 MPa indicates that the soil could reach a Ψof -50 kPa in treatments CT, NT11 and NT24; -88kPa in MTC3 and -200 kPa in MTC1, with no physicallimitations to root growth (Figure 4b-I). Consideringthe irrigation management monitored with atensiometer in clay soils under soybean, irrigationshould be initiated when the soil reaches a Ψ of-70 kPa (Guerra & Antonini, 1997). Collares (1994)

studied the performance of soybean under differentirrigation levels and noted that soybean yields werenot affected by a water deficit up to a Ψ of -150 kPain the 0.0-0.20 m layer.

In the 0.20-0.30 m layer and considering a SPRvalue of 2 MPa, the Ψ in NT24 was -15 kPa, but lessthan 10 kPa in the other tillage and cropping systems(Figure 4c-I,c-II). This indicates that θ should behigher than the field capacity to prevent a reductionof the root volume or even a barrier to root penetrationin the soil. In MTC3, NT11 and NT24, the SPR valuesreached 3.5 MPa for water tension below Ψ of -70 kPa.In treatment MTC1, a SPR value of 3.5 MPa wasassociated with a Ψ of -160 kPa. However, in treatmentCT, a Ψ of -20 kPa was obtained at SPR of 3.5 MPa(Figure 4c-I). This reduction in Ψ may be attributedto soil compaction in this treatment, that is, physicalrestrictions to plant growth and development occurredin this tillage system, resulting in a lower soybeangrain yield (656 kg ha–1) than in NT24 (Moraes, 2013).


Figure 4. Soil water matric potential when the soil penetration resistance reaches 2 and 3.5 MPa in the

layers 0.0-0.10 m (a); 0.10-0.20 m (b) and 0.20-0.30 m (c) for tillage (I) and cropping systems (II). Means

followed by the same letter, uppercase between tillage systems, and lowercase in the same tillage or

cropping system, do not differ by Tukey’s test (p<0.05). CT: conventional tillage; MT1: minimum tillage,

annual chiseling; MT3: minimum tillage, chiseled every three years; NT11: no-tillage for 11 years; NT24:

no-tillage for 24 years. Vertical bars indicate standard deviation from the mean.


R. Bras. Ci. Solo, 38:288-298, 2014

296

This indicates that the use of a SPR value of 3.5 MPacould be adequate for this soil under long-term NT orperiodic chiseling every three years.

The critical SPR limit of 2 MPa is a veryconservative threshold and not compatible with theresults of soybean and wheat grain yield (Figure 5).The soybean and wheat grain yield was notsignificantly related to SPR at Ψ of -33 kPa in the0.10-0.20 m layer, which is the most important layerfor soil physical quality (Debiasi et al., 2010), and inmost cases, the one with the highest degree ofcompaction in NT systems (Franchini et al., 2011).Nevertheless, we observed that, regardless of thetillage system, all SPR values were higher than 2MPa, and the yield in the CT plots was lower than inthe other tillage systems. The wheat and soybean grainyield was higher in NT24 than in CT, even at SPRvalues of 3 MPa. This indicates that the critical limitof SPR should be readjusted according to the tillagesystem. With the exception of soybean grain yield inCT, there were no yield losses due to physicalrestrictions to crop growth in the other tillage systems(Moraes, 2013). A comparison of these findings withpublished results showed that the soybean and wheatyields in this study were similar to the average grainyield of these crops in the 2011/12 growing season inthe State of Paraná, Brazil (CONAB, 2012). Inaddition, the soybean, corn and wheat yields in thisexperiment, monitored over two decades, werestabilized from the seventh year under NT onwards(Franchini et al., 2012).

Analyzing θ and Ψ in the three layers evaluated,it was confirmed that the adoption of a threshold of

3.5 MPa for SPR instead of 2 MPa would be suitablefor production systems under consolidated NT (NT11and NT24) or soil chiseling every three years (MTC3).It is important to mention that the critical thresholdSPR value of 3.5 MPa may not be suitable for systemswith intensive soil tillage, such as CT with heavyplowing and harrowing or annual soil chiseling(MTC1).

Also evaluating the SPR status at a PAW of 0.7(Carlesso, 1995; Santos & Carlesso, 1998), i.e., when30 % of the water-holding capacity (between fieldcapacity and permanent wilting point) was extractedfrom all three layers of the soil profile, the tillagesystems had a significant effect but not the croppingsystems (Figure 6).

In the 0.0-0.10 m layer, SPR (at PAW = 0.7) washighest in NT11 (2.7 MPa) and lowest in CT and MTC1with SPR of 0.95 and 1.4 MPa, respectively. In the0.10-0.20 m layer, the treatments NT11 and NT24 hadSPR values of 3.5 and 3.3 MPa (at PAW = 0.7),respectively. Also in this layer, the managementsystems with soil chiseling, MTC1 and MTC3, had SPRvalues of 2.5 and 3.0 MPa, respectively. For the 0.20-0.30 m layer, the SPR values were 3.1 and 3.3 MPain NT11 and NT24, respectively. However, in the samelayer, MTC1 and MTC3 resulted in SPR values of 2.8and 2.9 MPa, respectively (Figure 6). Carlesso et al.(1997) suggested a PAW value of 0.6 as critical limitto avoid reductions in leaf area. These authors foundthat sorghum plants began to wither at PAW below0.5. Therefore, to prevent conditions at which plantswould be subjected to water stress down to a depth of0.30 m of this soil type, the SPR values at PAW of 0.7

2.8

2.6

2.4

2.2

2.0

1.8

1.6

0.0 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6

Soil penetration resistance - SPR, MPa

Gra

in y

ield

, M

g h

a-1

CTr

MTC1sMTC1r

MTC3r

CTs

NT11s

MTC3sNT11r

NT24s

NT24r

Wheat = -0.72+2.38*SPR-0.44*SPR2

2R = 0.30; p = 0.29

Soybean = -6.14+6.25*SPR-1.15*SPR2

2R = 0.46; p = 0.11

Figure 5. Soybean and wheat grain yield as related

to soil penetration resistance at a matric

potential of -33 kPa in the 0.10-0.20 m layer under

the tillage systems: CT- conventional tillage;

MTC1- minimum tillage with annual chiseling;

MTC3 - minimum tillage with chiseling every

three years; NT11-no-tillage for 11 years; NT24 -

no-tillage for 24 years; r - crop rotation; s - crop

succession.


Figure 6. Soil penetration resistance of the tillage

and cropping systems when the fraction of

plant-available water reached 0.7. ns Not

significant in each layer by the F-test (p<0.05);

Means followed by the same letter, uppercase

between tillage systems, and lowercase in the

same tillage or cropping systems, do not differ

by Tukey’s test (p <0.05). CT: conventional tillage

system; MTC1: minimum tillage, annual

chiseling; MTC3: minimum tillage, chiseled every

three years; NT11: no-tillage for 11 years; NT24:

no-tillage for 24 years. Vertical bars indicate

standard deviation from the mean.


R. Bras. Ci. Solo, 38:288-298, 2014

should be adjusted, according to the tillage system.Considering the results of soybean and wheat grainyield over two decades (Franchini et al., 2012) and ofthe harvest of the 2011/2012 growing season (Moraes,2013), an increase in the critical SPR limits as afunction of tillage system is recommended. In thiscontext, critical SPR values up to 3 MPa could beused in management systems with annual chiselingor chiseling every three years (MTC1 and MTC3). Inconsolidated NT systems (NT11 and NT24), SPRthresholds up to 3.5 MPa can be applied. However, itis important to emphasize that in systems with heavytillage (CT), the critical SPR value of 2 MPa shouldnot be altered, because the results of the soybean grainyield indicated a yield loss in the NT24 treatment atthis SPR level (Moraes, 2013), resulting in waterrestrictions in the soil profile, and in CT systems theuse of the critical SPR at PAW of 0.7 may not beappropriate.

CONCLUSIONS

1. For the studied Rhodic Eutrudox with veryclayey texture under continuous no-tillage, the criticalvalue of soil penetration resistance should be increasedfrom the actual 2.0 to 3.5 MPa.

2. For systems under minimum tillage, withannual chiseling or chiseling every three years, on aRhodic Eutrudox with very clayey texture, the limitingvalue of soil penetration resistance should be increasedto 3 MPa.

3. However, for conventional tillage systems withannual tilling of a Rhodic Eutrudox with very clayeytexture, the critical value of soil penetration resistanceof 2 MPa should be maintained.

4. Soil penetration resistance was not altered bythe cropping systems, i.e., by the crop successionsystems: [wheat (Triticum aestivum L.)/soybean(Glicine max L.)] and/or crop rotation [white lupine(Lupinus albus L.) and radish (Raphanus sativus L.)/maize (Zea mays L.) - oat (Avena strigosa Schreb.)/soybean - wheat/soybean - wheat/soybean].

5. The positive effect of soil chiseling of a RhodicEutrudox with very clayey texture on soil penetrationresistance lasted at most 10 months in relation tocontinuous no-tillage. Thus, continuous no-tillagecould be the best management option for this soil type.

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